a) Field of the Invention
The invention is directed to an arrangement for the optical detection of a moving target flow for pulsed energy beam pumped radiation generation based on a plasma, for example, for the generation of extreme ultraviolet radiation (EUV), soft x-ray radiation or particle radiation.
b) Description of the Related Art
When intensive laser radiation interacts with material, soft-x-ray radiation, particularly EUV radiation, and particle radiation, can be generated under defined conditions. For this purpose, intensive laser pulses are conducted to a solid, liquid or gaseous material (target) and generate in the latter a plasma which emits the desired radiation. When liquids are used as target material and introduced into an evacuated interaction chamber by a target generator, these moving targets must advantageously be excited identically as far as possible by the high-energy excitation beam in an advantageous manner. Only in this way can an efficient and stable radiation be generated.
WO 02 11 499 A1 discloses a method for the generation of x-ray radiation or EUV radiation in which an electron beam is made to interact with a moving target jet in a vacuum chamber. In this case, in order to adjust the desired type of radiation—soft x-ray radiation or EUV radiation—the electron beam that is used is directed to a liquid target flow that is ejected from a pressure chamber through a nozzle for generating a plasma. This solution provides no information about the wavelength stability and energy stability of the radiation which is accordingly insufficiently defined for exposure processes in semiconductor fabrication.
Therefore, in order to stabilize radiation generation, another solution was suggested in WO 02 32 197 A1 in connection with the generation of EUV radiation. This solution involves regulation based on a temperature measurement of the outlet nozzle of the liquid jet.
The solutions described above share the disadvantage that the position of the target flow during plasma excitation by high-energy radiation (e.g., a laser beam or electron beam) is not monitored, so that variations in emissions occur due to the different location of the targets. This can not be tolerated, e.g., in photolithography exposure machines.
Further, it is known from the prior art to use a continuous transmitted emission and a time-variable return emission of moving objects or of objects with variable reflectivity. For example, for the purpose of determining the position of drops in inkjet printing technology, U.S. Pat. No. 4,510,504 describes a device for optical determination of the position of a drop in which the light of a light-emitting diode which is reflected by the drop reaches a photodetector. This arrangement is so constituted that the drop reflects light in the direction of the detector and accordingly generates a signal only at a defined position. An arrangement of this kind is obviously not suitable for detection of the drop position in a vacuum chamber in plasma generation for x-ray generation because it detects the scatter light of the energy beam used for plasma generation along with the radiation emitted by the plasma, so that precise measurement is not possible. In addition, the active electronic components are influenced in an impermissible manner when radiation is generated in the vicinity of the plasma due to the extreme environmental conditions (for example, hard x-ray radiation with high intensity or neutron radiation) and their useful life is considerably diminished.